Compliant structures for use in robots are well known as shown in U.S. Pat. Nos. 4,571,148--Drazan; 4,518,307--Bloch; 4,447,048--Fischer; 4,439,926--Whitney; and 4,327,496--Rebman. Manipulators generally are designed for end-point position control, and their controls have evolved from numerical control techniques pioneered for milling machines and lathes. The essence of the machine tool is the ability to hold the position of the tool independently of the force; the part produced by the machine tool is the same regardless of the size of the chip removed. With the emphasis on position control, limbs are made stiff and are bulky; rigidity being required to hold position independent of the load, and to insure that resonant bending frequencies are beyond the response of the control system. Current robot manipulators are capable of handling payloads on the order of 5% of the robot weight, which value is low compared, for example, to earth moving equipment, factory or construction cranes, and animals. Such a low payload-to-weight has several unfavorable implications. Heavier manipulators are in general more expensive to build and to operate. Not only is more construction material used, but expensive items such as bearings and motors must be larger. Since braking usually is not regenerative, the energy used in manipulator motion, by far the most of which is for motion of the manipulator itself, is lost. Larger manipulators require larger foundations and are less likely to be put on mobile bases. Also, larger manipulators are likely to be more dangerous, simply because of their size.
Compliant structures, such as those shown in the above-mentioned patents, often are used for force control and constrained motion tasks. Generally, compliance is added at or near the end effector of the manipulator. Similarly, hammering has also been done by adding a device at the end of the manipulator that performs the action rather than the manipulator itself. If the manipulator included the necessary compliances, then there would be no need to add compliance structure thereto, and an improvement in payload-to-weight could be realized.
Control systems which take into account compliance in a manipulator arm for increased bandwidth also are known as shown in U.S. Pat. No. 4,603,284--Perzley. Often, the manipulator structure with which the control system is used is not designed to include compliance, the compliance being inherent in the machine structure.
Other robotic manipulators with compliance in the joints also are known. Although compliance in the joints could refer to stiffness of the drive train, it generally is taken to be backdrivability so that the stiffness of the joint can be controlled by the actuator. The usual direction of actuation is for the actuator to move the joint. When the joint can be moved by external forces and move the actuator, then the joint is backdrivable. (Worm gear transmissions are frequently nonbackdrivable, and can be designed to be nonbackdrivable. High gear ratio transmissions in general are nonbackdrivable, and in the above-mentioned sense of the word, hydraulic systems and stepping motor drives also are nonbackdrivable.) Low gear ratio and direct drive electric motors are examples of backdrivability.
The historical trend has been from manipulators designed with rigid limbs and non-backdrivable joints toward the use of manipulators with rigid limbs and backdrivable joints. The well known SCARA (Selective Compliance Assembly Robotic Arm) configuration is of the latter type. Such a configuration is shown, in the publication by Makino, H and N. Furuya (1980), "Selective Compliance assembly Robotic Arm," Proceedings of the First International Conference on Assembly Automation, pp. 77-86. UFS Ltd. Bedford. A key, however, to the SCARA design is the use of vertical axis joints so that the motors are not gravity loaded. This allows the motors to be reasonable in size and minimizes power required to hold position.
Experimental robots having the combination of a compliant limb and backdrivable joints are known as shown in the following publications: Book, W. J., O. Maizza-Neto, D. E. Whitney (1975), "Feedback Control of Two Beam, Two Joint Systems with Distributed Flexibility," ASME J. of Dynamic Systems, Measurement, and Control, Vol. 97, No. 2, pp. 424-431; Dubowsky, S., T. N. Gardner (1977), "Design and Analysis of Multi-Link Flexible Mechanism with Multiple Clearance Connections," ASME J. Engineering for Industry, Vol. 99, No. 1; and Hennessey, M. P., J. A. Priebe, P. C. Huang, and R. J. Grommes (1987), "Design of a Lightweight Robotic Arm and Controller", Proceedings of the 1987 IEEE Conference on Robotics and Automa. Such robots provide for reduced weight while retaining much control capability through the joint actuators.
Also, the broad idea of a robot having compliant limbs and nonbackdrivable joints is known as demonstrated by W. J. Book at the Army Research Office workshop on Kinematics, Dynamics, and Control of Mechanisms and Manipulators, Rensselaer Polytechnic Institute, June, 1986. A position control scheme for use on a single nonbackdrivable joint compliant arm was described by Andeen, G. B., and C. M. Dickinson (1986), "Torque Programming," at the ARO Conference on Kinematics, Dynamics and Control of Mechanisms and Manipulators, RPI, June.
Several researchers have successfully developed control schemes to control the endpoint position of a single flexible link, which schemes are described in the following publications: Balas, M. J. (1978), "Feedback Control of Flexible System," IEEE Trans., Vol. AC-23, No. 4, pp. 673-679; Cannon, R. H. Jr., E. Schmitz (1984), "Initial Experiments on the End-Point Control of a Flexible One-Link Robot," International Journal of Robotics Research, Vol. 3, No. 3, pp. 62-75; Hastings, G. G., and W. J. Book (1985), "Experiments in the Control of a Flexible Robot Arm," Robots 9 Conf. Proc., June, Detroit, Vol. 2, pp. 20-45 to 20-57; De Maria, G. and B. Siciliano (1987), "A Multilayer Approach to Control of a Flexible Arm," Proceedings of the 1987 IEEE Conference on Robotics and Automation, Raleigh, NC., April, pp. 774-778; Wang, D. and M. Vidyasagar (1987), "Control of a Flexible Beam for Optimum Step Response," Proceedings of the 1987 IEEE Conference on Robotics and Automation, Raleigh, NC., April, pp. 1567-1572; and Shung, I. Y. and M. Vidyasagar (1987), "Control of a Flexible Robot Arm with Bounded Input: Optimum Step Responses," Proceedings of the 1987 IEEE Conference on Robotics and Automation, Raleigh, NC., April, pp. 916-922.
Position control schemes for industrial manipulators having multi-degrees of freedom considering only small deflections are described in the following publications: Truckenbrodt, A. (1979), "Dynamics and Control Methods for Moving Flexible Structures and Their Application to Industrial Robots," Proceedings 5th World Congress on Theory of Machines and Mechanisms--Vol. 1, ASME, New York, pp. 831-834; Futami, S., N. Kyura, S. Hara (1983), "Vibration Absorption Control of Industrial Robots by Acceleration Feedback," IEEE Trans. On Industrial Electronics, Vol. IE-30, No. 3, pp. 299-305; and Amazigo, G. O. (1984), "Forced Three Dimensional Motion of a Deformable Arm," ASME Technical Paper #84-DET-124, ASME, NY, NY.